1. Field of the Invention
This invention relates to an improvement in a vane type hydraulic pump which comprises a rotor, a stator, vane slots in one of the rotor or stator, and vanes in the vane slots which sequentially traverse an intake or suction zone, a transfer zone, a pressure or exhaust zone and a sealing zone when the pump is operated. More specifically, the invention relates to an improved vane structure for a pump which incorporates an extended undervane suction port.
2. Description of the Prior Art
Conventional two-lip, solid vanes are well known in the art and are commonly used in vane pumps. As these vanes traverse the transfer zone on the major diameter of the cam ring, the leading face is exposed to high pressure fluid from the pressure port and the pressure fluid tends to leak across the leading lip or edge of the vane. This leakage creates a high pressure on top of the vane which tends to bias the vane inwardly of the vane slot and away from the cam ring. Separation of the vane from the cam ring is potentially disastrous, since it provides a short circuit path for high pressure fluid to reach the low pressure zone behind the vane.
To overcome the force acting to bias the vane inwardly as it traverses the transfer zone, springs or hydraulically operated pins are placed in the bottom of the vane slot to bias the vane outwardly. A problem with using a spring or a hydraulic pin to bias a solid vane is that, when the pressures on the inner and outer ends of the vane are equal, as when it traverses the pressure and suction zones, the force exerted by the spring or pin is considerably in excess of what is needed to maintain the vane in contact with the cam ring. As a result, the edges of the vane and cam ring wear at a relatively high rate.
To overcome some of the problems associated with a solid two-lip vane, passages were formed in the vane, which passages connected the inner and outer ends of the vane. Such passages are shown in U.S. Pat. No. 3,359,914 to Adams, which is assigned to the assignee of the instant invention. The purpose of the passages is to equalize or balance the pressure on the inner and outer ends of the vane at all times. With a balanced vane a reduced spring or pin force is required to maintain a vane in contact with the cam ring. This greatly reduces vane and cam ring wear.
The above-mentioned Adams' patent also purported to solve the problem of reduced pump efficiency caused by high pressure fluid leaking past the leading edge of the vane as it traversed the transfer zone. In Adams it is theorized that this leakage fluid produces a turbulence in the low pressure zone immediately behind the vane which adversely affects the filling of the intervane space. In order to reduce this turbulence, high pressure fluid which leaks over the leading edge is forced through a tortuous path formed in the vane to reduce the velocity of the fluid. Subsequently, the low velocity fluid is routed to the back of the vane.
Balanced vanes which permit the free flow of fluid between the inner and outer ends encounter difficulty when the pump is run at relatively high speeds. When the vane traverses the pressure zone it is moved inwardly in the vane slot by the inwardly inclined pressure ramp on the cam ring. Since the inner and outer ends of the vane are connected, the inward movement of the vane is not resisted by fluid pressure under the vane. It is only resisted by the springs or pins under the vane. At high speeds the momentum of the vane is such that the vane continues to travel inwardly after it reaches the end of the pressure ramp. Thus, it skips or loses contact with the initial portion of the minor diameter. The vane is subsequently snapped back against the cam ring by the springs or pins, but wear of the cam ring and vane edges is increased from the vane striking the cam ring.
One way to prevent the vane from skipping at the end of the pressure ramp is to provide very heavy spring or pin forces to bias the vane outwardly. These heavy forces will, however, cause accelerated wear of the vane edges and the cam ring.
The tendency of vanes to move inwardly past the end of the pressure ramp at high pump speeds and skip the initial portion of the minor diameter was substantially reduced by the improvements shown in U.S. Pat. No. 3,781,145 to Wilcox, which is assigned to the assignee of the instant invention. Wilcox discloses inserting a flow restricting orifice in the fluid passage which connects the inner and outer ends of the vane. Inward movement of the vane in its rotor slot as the vane traverses the pressure zone causes fluid to be displaced from the inner end of the vane slot through the orifice to the outer end. As the fluid flows through the orifice, a pressure differential arises and an outward directed force is created at the inner end of the vane which resists inward movement of the vane. Consequently, the pump can operate at higher speeds before the vane begins to skip at the end of the pressure ramp.
Vanes with the flow restricting orifices, described in the Wilcox patent, have been used in vane pumps having extended undervane suction ports which are described in U.S. Pat. No. 3,790,314 to Swain and assigned to the assignee of the instant invention. The purpose of an extended undervane suction port is to provide fluid communication between the inner end of each vane slot, angularly, beyond the end of the main suction port. Ideally, the undervane suction port extends toward the pressure port to a position just short of the point where it would provide a short circuit connection between the pressure and suction ports. The primary advantage of the undervane suction port is that it provides improved filling of the intervane pocket and enables the pump to operate satisfactorily at higher speeds.
In a pump which has extended undervane suction ports, the top of the vane is beyond the suction port while the bottom of the vane remains connected to the undervane suction port for a number of degrees of travel as the vane traverses the transfer zone. Shortly after passing the suction port and while the bottom of the vane is connected to the undervane suction port, the vane is exposed to high pressure fluid. During this time, any pressure fluid which leaks across the leading edge of the vane will flow inwardly through the vane past the restrictors, since the fluid at the outer end of the vane is at higher pressure than the fluid at the inner end. As the fluid flows through the orifice, a pressure differential arises and an inwardly directed force is created which tends to move the vane inwardly of its slot and away from the cam ring. This inward force lasts until the inner end of the vane is disconnected from the undervane suction port.
The problem associated with the extended undervane suction port could be eliminated by removing the extended undervane suction port. However, this is unacceptable since it provides a tremendous increase in the performance of the pump. Another solution is to take the orifice out of the passage which connects the inner and outer ends of the vane. This is undesirable since the advantage of the dash pot effect at the end of the pressure ramp which enables the vane to track the cam ring at higher pump speeds is lost. Of course, stronger springs or increased pin force could be resorted to, but these increase the wear of the vanes and cam ring.
Consequently, it is desirable to provide a pump which includes extended undervane suction ports and vanes with restricted passages which connect the inner and outer ends, which do not require large spring or pin forces and which do not permit pressure fluid leaking across the leading edge of the vane to push the vane away from the cam ring during the time the bottom of the vane is connected to the undervane suction port and the top of the vane is exposed to high pressure fluid.